Metal and metal oxide co-functional single-walled carbon nanotubes for high performance gas sensors

ABSTRACT

A method of co-functionalizing single-walled carbon nanotubes for gas sensors, which includes the steps of: fabricating single-walled carbon nanotube interconnects; synthesizing tin oxide onto the single-walled carbon nanotube interconnects; and synthesizing metal nanoparticles onto the tin oxide coated single-walled carbon nanotube interconnects.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application claims priority to U.S. Provisional PatentApplication Ser. No. 61/346,104, filed May 19, 2010, which isincorporated herein by reference in its entirety.

GOVERNMENT INTEREST

This invention was made with Government support under Grant No. ES016026awarded by the National Institutes of Health. The Government has certainrights in this invention.

FIELD OF THE INVENTION

This invention relates to a method of co-functionalizing single-walledcarbon nanotubes (SWNTs) for gas sensors, and more particularly to asensor having co-functionalized single-walled carbon nanotubeinterconnects formed by synthesizing tin oxide onto single-walled carbonnanotube interconnects, and synthesizing metal nanoparticles onto thetin oxide coated single-walled carbon nanotube interconnects.

BACKGROUND

The applications of SnO₂-SWNTs (tin oxide-single-walled carbonnanotubes) hybrid structure as room temperature gas sensing platform hasbeen demonstrated in previous studies. Combining efficient transductionproperty of SWNTs with high molecular detection property of SnO₂,excellent sensitivities to trace quantities of both oxidizing (limit ofdetection (LOD) of 25 ppb for NO₂) and reducing gases (LOD of 10 ppm forH₂) at room temperature was observed. The enhanced sensing performanceobserved for these hybrid nanostructures compared to unfunctionalizedcarboxylated SWNTs, is attributed to the availability of increasedsurface area of active elements, which can take part in gas moleculeinteractions. Although promising results were observed for these hybridnanostructures, further improvement in sensitivity and particularlyselectivity towards specific analytes remains a challenge.

Typically, sensors using SnO₂ as a sensory element use small amounts ofadditives such as Pd, Pt, Au, Ag, etc., to increase sensitivity andselectivity towards specific analytes. Generally, two differentmechanisms have been considered to explain the observed enhancement insensing performance for metal particles impregnated tin oxide sensors.The first is called chemical sensitization, where the metal particlescatalytically activate the redox processes occurring at the tin oxidesurfaces by lowering the activation energy for dissociation of analytegases such as O₂, H₂, H₂S, CO, etc. The activated products then migratetowards the tin oxide surface, to react with adsorbed oxygen speciesresulting in a greater and faster degree of charge transfer between tinoxide and the adsorbate. The second mechanism is called electronicsensitization, where the metal nanoparticles interact electronicallywith the tin oxide surface forming charge depletion zones around theparticles. Any changes observed in the work function of the additive dueto gas adsorption and desorption will cause a change in the Schottkybarrier between the metal particle and tin oxide resulting inconductivity changes. The two processes are schematically represented inFIG. 1.

SUMMARY

In accordance with an exemplary embodiment, a novel approach for highperformance gas sensors using metal nanoparticles and SnO₂co-functionalized single-walled carbon nanotubes has been developed. Inan exemplary embodiment, a sequential electrochemical templating methodwas employed where SWNTs were first functionalized with tin oxide usingelectrochemical assisted approach, followed by electrodeposition ofmetal nanoparticles on top of SnO₂. Three different noble metalcatalysts were selected for sensing studies such as palladium (Pd),platinum (Pt) and gold (Au). The sensors were tested towards differentcombustible and toxic gases (NH₃, NO₂, H₂, H₂S, water vapor) at roomtemperature. Among the fabricated sensors, Pd decorated SnO₂-SWNTsshowed pronounced effects on introduction to target analytes withextremely high sensitive response obtained towards H₂S (500% resistancechange for 5 ppm). This remarkable improvement in sensing performanceobserved for Pd decorated SnO₂/SWNTs can be attributed to the formationof electro active elements on the surface of metal oxide-SWNT hybridstructures and enhanced catalytic decomposition of interacting gases onPd nanoparticle surfaces.

In accordance with an exemplary embodiment, a method ofco-functionalizing single-walled carbon nanotube interconnects for gassensors comprises: fabricating single-walled carbon nanotubeinterconnects; synthesizing tin oxide onto the single-walled carbonnanotube interconnects; and synthesizing metal nanoparticles onto thetin oxide coated single-walled carbon nanotube interconnects.

In accordance with another exemplary embodiment, a sensor havingco-functionalized single-walled carbon nanotube interconnects formed bysynthesizing tin oxide onto the single-walled carbon nanotubeinterconnects, and synthesizing metal nanoparticles onto the tin oxidecoated single-walled carbon nanotube interconnects.

In accordance with a further exemplary embodiment, a sensor comprises: apair of microelectrodes; and a tin oxide coated single-walled carbonnanotube interconnects, which extends across a gap between the pair ofmicroelectrodes, and wherein the tin oxide coated single-walled carbonnanotube interconnects include metal nanoparticles on top of the tinoxide coated single-walled carbon nanotubes.

The details of one or more embodiments of the disclosure are set forthin the accompanying drawings and the description below. Other features,objects, and advantages will be apparent from the description anddrawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention. In the drawings,

FIG. 1 is a schematic representation of chemical and electronicsensitization processes for noble-metal catalyst loaded SnO₂ gas sensor.

FIG. 2 is a schematic representation of sequential electrochemicaltemplating approach for fabricating hetero-nanoarchitectures.

FIG. 3 are SEM images of a) carboxylated SWNTs, b) SnO₂ coated SWNTs andc) Pd nanoparticles decorated SnO₂/SWNT heterostructure, with theircorresponding EDX spectrums (d-f), and with low SEM magnification imagesof the devices shown in the inset.

FIG. 4 is an image of a) resistance change observed after Pdnanoparticle decoration on SnO₂/SWNTs, and wherein device resistancesafter SnO₂ decoration varied from 5 Kohms to 10 Mohms, the inset showscorresponding I-V curve obtained for a single device, and b) FETmeasurements of SnO₂/SWNT device and Pd decorated SnO₂/SWNT device.

FIG. 5 are schematic depictions of the two major processes which mayoccur on Pd nanoparticle decoration on SnO₂/SWNT compound structure,wherein (1) shows oxidation of Pd to PdO by the ionosorbed oxygenspecies present on the tin oxide resulting in electron transfer acrossSnO₂/SWNT interface and (2) shows Pd nanoparticle deposition on SWNTsresults in formation of a depletion region creating obstruction for holetransport and therefore decreases carrier mobility.

FIG. 6 is a sensor response of SWNTs, SnO₂/SWNTs and Pd/SnO₂/SWNTstowards different concentration pulses of H₂ (10, 50, 100 and 250 ppm).

FIG. 7 are a) histogram showing sensor performance of SWNTs, SnO₂/SWNTsand Pd/SnO₂/SWNTs towards NH₃, NO₂ and H₂S, and b) real-time sensingresponse of SWNTs, SnO₂/SWNTs and Pd/SnO₂/SWNTs towards 5 ppm H₂Sconcentration, and wherein the insets are magnified responses for SWNTsand SnO₂/SWNT sensors.

FIG. 8 is a comparison of sensing performance towards 5 ppm H₂S gas foras fabricated Pd/SnO₂/SWNTs with that of same sensor after 23 days ofstorage.

FIG. 9 is an optical and SEM images of a nano gas sensor array usingSWNTs functionalized with different sensing elements, and wherein theinset shows low magnification SEM images of the same (all scale bars inthe inset correspond to 1 μm).

FIG. 10 is a sensor array response to different concentrations of NH₃.

FIG. 11 is sensor array response to different concentrations of H₂S.

FIG. 12 is a sensor array response to different concentrations of NO₂.

FIG. 13 is a sensor array response to different percentages of watervapor.

FIG. 14 is a histogram showing response of each gas sensor element toPEL concentration of NH₃ (25 ppm), NO₂ (5 ppm), H₂S (20 ppm) and 50% ofrelative humidity.

DETAILED DESCRIPTION

The synthesis of ternary hybrid nanostructures by electrode positingmetal nanoparticles on SnO₂-SWNT hybrid structure has been successfullydemonstrated in accordance with an exemplary embodiment as set forth. Inaccordance with an exemplary embodiment, these nanostructures can beused as chemiresistors to compare the chemical sensing performancetowards various analytes. Compared to SnO₂/SWNT hybrid nanostructures,Pd/SnO₂/SWNTs displays highly improved response towards all the analytestested. Especially, Pd functionalized SnO₂/SWNT ternary nanostructuresshowed dramatic improvement in sensitivity for H₂S gas. The dual role ofPd as a catalytic activator and Schottky barrier modulator may beattributed for the observed dramatic enhancement in sensitivity towardsH₂S.

In accordance with an exemplary embodiment, experimental results arepresented for SnO₂-SWNT gas sensors functionalized with different metal(Pd, Pt and Au) nanoparticles upon exposure to different analytes. Theexperimental results presented here are an excellent source fordeveloping a reference response spectrum that can be later used torecognize a particular gas species.

It can be appreciated that in comparison with other techniques such aselectron beam evaporation, self-assembly, sol-gel synthesis etc.,electrodeposition offers a simple route to fabricate heteronanostructures. Moreover, electrodeposition offers good control on thenumber and distribution of metal nanoparticles, which may be vital intailoring the functionalities of such compound structures towardsparticular analytes.

Assembly of SWNTs:

In accordance with an exemplary embodiment, sensor arrays weremicrofabricated by the following methodology. Microelectrodes werepatterned on SiO₂/Si substrates using photolithography followed byelectron beam evaporation of Cr/Au electrodes. To fabricate SWNTinterconnects across the electrodes; first, carboxylated-SWNTs(SWNT-COOH 80-90% purity) (Carbon Solution, Inc. Riverside, Calif., USA)were dispersed (10 μg/mL) in dimethyl formamide (DMF, Sigma Aldrich,Mo., USA) using ultrasonic force for 60 min. Then, a 50 nano-liter ofthe SWNT solution was aligned between the gold electrodes using ACdielectrophoretic technique. AC dielectrophoresis (DEP) uses anon-uniform electric field, to align and to move the suspended nanotubesbundles towards the electrode. A 4 MHz ac field with 3 V peak to peakfor 10 seconds was found to be optimal for alignment of SWNTs. Followingalignment, the electrodes were rinsed with nanopure water and dried withnitrogen air. The sensors were then annealed at 300° C. for 60 minutesunder reducing atmosphere (5% H₂+95% N₂) to minimize the contactresistance between the aligned SWNT network and the gold pads and toremove any DMF residues present. The number of SWNTs bridging theelectrode gap was controlled by either adjusting the concentration ofthe SWNTs in the DMF solution or by adjusting the alignment time.

Synthesis of SnO₂ Coated SWNTs:

The electrolyte solution, for templating tin oxide on SWNTs was preparedaccording to the work of Min et al. (See Lai, M., et al.,Size-controlled electrochemical synthesis and properties of SnO2nanotubes. Nanotechnology, 2009. 20(18): p. 185602, which isincorporated herein in it entirety). First 100 mM of NaNO₃ (≥99.0%,Sigma-Aldrich, Mo.) is added to 75 mM of HNO₃ (70%, Sigma-Aldrich, Mo.)under constant stirring. 20 mM of SnCl₂5H₂O (≥98%, Sigma-Aldrich, Mo.)was then added, and the solution (pH of approximately 1.3) was aged for12 hours under constant stirring prior to use. All depositions werecarried out in potentiostatic mode (constant potential) at 25° C.Chronoamperometry measurements were carried out in a three electrodeelectrochemical setup using a commercial potentiostat (EG&G, PrincetonApplied Research 263A Potentiostat/Galvanostat) with aligned SWNTs asworking electrode and Pt wire (99.99%, Sigma-Aldrich, Mo.) and saturatedAg/AgCl wire as auxiliary and reference electrodes. Electrochemical cellwas formed by dispensing a 3 μL of electrolyte solution on top of thealigned SWNT network and platinum and Ag/AgCl wires were positionedinside the droplet using micropositioner. After deposition process theelectrodes were rinsed with deionized water to remove any metal saltresidues and impurities present.

Synthesis of Metal Nanoparticles on SnO₂ Coated SWNTs:

Electrodeposition of metal nanoparticles on the SnO₂ coated SWNTnetworks were performed using a three electrode electrochemical cellconfigurations. For palladium deposition 10 g/L of Pd (NH₂)₂(NO₂)₂ and100 g/L of ammonium sulfamate was first added. The solution pH was thenadjusted to 8.0 by addition of sulfamic acid and sodium hydroxide. Forgold and platinum deposition, commercially available ready-to-useelectroplating solution from Technic Inc (CA) was used as electrolyte.All aqueous solutions were prepared using nanopure water. For alldepositions, alkaline electrolyte solutions are selected (pH ofapproximately 7.5 to approximately 8.0) to prevent the dissolution of Cradhesion layer which can be readily attacked in an acidic environment.In accordance with an exemplary embodiment, SnO₂ coated SWNTs served asworking electrodes with Pt wire and Ag/AgCl wire as counter andreference electrodes respectively. High-resolution field emissionscanning electron microscopy (FE-SEM) and energy dispersive x-rayspectroscopy (EDX) was used as a characterization tool for the depositedsamples.

Gas Sensing Studies:

For gas sensing studies, the sensors were wire-bonded and each sensorwas connected in series with a load resistor. A 3.6 cm³ sealed glasschamber with gas inlet and outlet ports for gas flow-through waspositioned over the sensor chip. All experiments were conducted withdesired analyte gas (purity: 99.998%) diluted in dry air (purity:99.998%) at a gas flow of 200 std. cm³ min⁻¹. The analyte and dry airgas flow rates were regulated by mass flow controllers (AlicatScientific Incorporated, Tucson, Ariz., USA). A custom Lab view computerprogram was developed to continuously control and monitor the voltage ofthe circuit using field point analog input and output modules (NationalInstruments, Austin, Tex., USA). In all the experiments, sensors werefirst exposed to air to obtain the baseline, then to a desiredconcentration of analyte gas, and then back to air, which completed onecycle. This process was repeated for different concentration of analytebeing tested. All results shown here is a representation of 5 sensors ormore.

Nanobuilding Blocks Using Sequential Templating Approach:

In order to construct hybrid/hetero nano-architectures for gas sensingapplications, tin oxide coated SWNTs were first synthesized. As reportedin previous studies, electrochemical assisted approach was used totemplate discrete tin oxide nanocrystallites on the surface of SWNTs.Aligned SWNTs between gold electrodes served as the working electrode.On applying a suitable cathodic potential (−0.4 V vs. Ag/AgCl wire),nitrate ions were reduced on the surface of SWNTs producing hydroxylions which increased the local pH initiating the chemical precipitationof tin oxy-hydroxide on SWNTs, which was later converted to tin oxidenanocrystallites during post annealing process. Palladium nanoparticleswere then electrodeposited on the SnO₂-SWNT hybrid nanostructures. ForPd deposition, chronoamperometry technique with a constant depositionpotential of −0.8 V vs. Ag/AgCl wire was used. The deposition charge wasfixed to 5 μC. FIG. 2 shows a schematic representation of sequentialelectrochemical templating approach to form metal and metal oxideco-functionalized SWNTs.

FIGS. 3a, 3b, and 3c show SEM images of carboxylated SWNTs, SnO₂ nanocrystallites coated SWNTs and Pd nanoparticles decorated SnO₂-SWNTnanostructures, respectively. SEM observations (FIG. 3b ) reveal thatthe tin oxide nanocrystallites are coated homogenously all along thesurface of SWNTs. In addition to a homogenous surface observed forSnO₂-SWNTs, the nano-architectures consist of uniformly loaded palladiumnanoparticles onto the surface of SnO₂-SWNTs (FIG. 3c ) afterelectrodeposition of Pd. The EDX analysis (FIG. 3f ) confirmed thepresence of Pd, Sn, C and O, which substantiates that SnO₂-SWNT hybridnanostructures have been successfully modified with Pd nanoparticlesusing sequential electrochemical templating approach. No correspondingpeaks of Pd and Sn are observed for carboxylated SWNTs (FIG. 3d ) and noPd peak was observed for SnO₂ coated SWNTs (FIG. 3e ).

The electronic interaction between palladium nanoparticles and SnO₂-SWNThybrid nanostructure was investigated using room temperature conductanceand field effect transistor (FET) measurements (FIG. 4). It should benoted that all electrical transport measurements were done at ambientconditions. Two important electrical transport characteristics areobserved for palladium decorated SnO₂-SWNT device. First, there is adramatic decrease in conductance on palladium nanoparticlefunctionalization. FIG. 4a shows the change of resistance observed afterPd nanoparticle decoration on SnO₂/SWNT hybrid structure.

The initial resistances of the SnO₂/SWNT device were varied from 5 Kohmsto 10 Mohms. For all resistance ranges, introduction of palladiumnanoparticles increased the device resistance with a decrease insource-drain current observed for all gate voltages with no gatedependency (FIG. 4b ).

The increase in resistance after Pd nanoparticle deposition implies thatthere is a net electron transfer from palladium nanoparticles to theSnO₂-SWNT hybrid nanostructure device. As electrochemical assistedapproach for templating tin oxide on SWNTs produces discrete tin oxidenanocrystallites, electrodeposition of palladium nanoparticles on bothSWNT surface and on SnO₂ nanocrystallites deposited on SWNTs. Thepalladium nanoparticles decorated on surface of SWNTs act as chargescattering sites decreasing device mobility and hence conductivity asreported in our previous studies and also substantiated by our FETmeasurements. For palladium nanoparticles decorated on SnO₂nanocrystallites, there could be rapid oxidation of palladiumnanoparticles by ionosorbed oxygen species present on the surface ofSnO₂. The oxidation of palladium results in a net electron transfer intothe SnO₂ conduction band, which further results in electron transferacross SnO₂-SWNT interface, resulting in a decrease in conductance forp-type SWNTs. Hence, it can be appreciated that the main processes thatproduce the conductance changes are the removal of ionosorbed oxygenspecies by reaction with palladium nanoparticle adsorbate and creationof nano-Schottky barriers associated with palladium nanoparticleformation on SWNT surface. The two processes are schematicallyrepresented in FIG. 5.

Gas Sensors Constructed with Pd/SnO₂/SWNTs Nanoarchitectures:

The sensing performance of SWNTs, SnO₂/SWNTs and Pd/SnO₂/SWNTs towardsdifferent hydrogen concentration pulses are shown in FIG. 6.Functionalizing the SWNT surface with SnO₂ nanoerystallites enhances thesensing performance towards trace quantities of hydrogen concentration(limit of detection (LOD)—10 ppm). However, functionalizing these hybridnanostructures with Pd leads to an enhancement in sensor performance(which is evaluated as percentage change in resistance,ΔR/R0=Rgas/Rair⁻¹) almost double compared to SnO₂/SWNT samples.

The responses of Pd/SnO₂/SWNTs sensors (FIG. 7a ) to all of the gasestested here are higher than those of sensors fabricated fromcarboxylated SWNTs and SnO₂ coated SWNTs. Most remarkable is that thepalladium decorated SnO₂-SWNT hybrid structure showed very highsensitivity towards H₂S, even when the concentration of H₂S (5 ppm) waslower than the concentrations of other analytes tested. The sensingperformance of all three nanoarchitectures used in this study towards 5ppm H₂S is shown in FIG. 7 b.

For H₂S gas, the sensing response of Pd/SnO₂/SWNT structure is around600% for 5 ppm concentration, compared to 50% obtained for SnO₂/SWNTsand 5% obtained for carboxylated SWNTs. It can be appreciated that aresponse of this magnitude (600% for 5 ppm H₂S) observed forPd/SnO₂/SWNT sensor at room temperature has not been reported elsewhereusing any sensor element.

It can be appreciated that two processes are considered to explain theenhancement in sensing performance observed for palladium catalysts ontin oxide-SWNT sensors. The first process is called “electronicsensitization”, where the metal catalyst added creates an energybarrier, which depends primarily on the work function of the metalcatalysts used. As work function of palladium (5.15 eV) is higher thanthe SnO₂ (4.7 eV) and SWNTs (4.7-4.9 eV), a charge depletion zone isformed around the particles. Here the sensing performance attributed tothe modulation of these nano-Schottky barriers on introduction of theanalyte gas. The second process is called “chemical sensitization” wherethe catalytic activity of Pd nanoparticles is utilized. Here theenhancement is due to a “spill over” effect, in which the gas moleculesdissociate on the catalytic palladium particles and then diffuse acrossand/or through the particles reaching the SnO₂ surface resulting inconductance change. Considering the improvement observed at roomtemperature operation, the enhanced sensing performance observed forPd/SnO₂/SWNT H₂S gas sensor can be attributed to both the processesmentioned above. Hence, the significant decrease in conductance resultedfrom: 1) modulation of the nano-Schottky barriers due to the changes inthe work function of the palladium nanoparticles on introduction of H₂Sgas. H₂S gas can lower the work function of the palladium metalresulting in easy electron transfer from metal nanoparticle to SWNTsand/or to SnO₂ surface; and 2) palladium nanoparticles can lower theactivation energy for decomposition of H₂S gas which could facilitatethe diffusion of activated products (H and SH) towards tin oxidesurface. The activated products can then react with oxygen atoms on thetin oxide surface leaving behind oxygen vacancies, which serves aselectron donors, hence decreasing device conductance.

Another important factor for evaluating nanosensor performance is thestability and reproducibility of the device. The stability of thePd/SnO₂/SWNT sensor was examined by comparing its response to 5 ppm H₂Sobtained immediately after sensor fabrication to response obtained after3 weeks. The sensors were stored in a dessicator in the correspondingtime period. As illustrated in FIG. 8, the responses of the Pd/SnO₂/SWNTsensor showed no obvious degradation, suggesting its long-term stabilityand its potential for real time applications.

Construction of Nano Sensor Array:

In accordance with an exemplary embodiment, a novel hybrid chemicalsensor array composed of aligned carboxylated SWNTs network, metalnanoparticle (Pd, Pt and Au) decorated SWNT network, tin oxide decoratedSWNT network, metal catalyst (Pd, Pt and Au) impregnated SnO₂-SWNTnetwork was fabricated. A key feature of this approach was usingelectrodeposition for fabricating all the sensor elements in one singlechip. FIG. 9 shows an optical image of the sensor chip used and SEMimages of all the sensor elements employed. For Pt and Au decoration onSnO₂/SWNT hybrid structure a deposition potential of −1.0 V vs. Ag/AgClwire was used for a constant charge of 5 μC.

The eight element sensor array was then exposed to different analytessuch as NH₃, NO₂, H₂S and water vapor at different concentrations. FIGS.10-13 show real-time gas sensing performance of different elementstowards different analytes.

The histogram in FIG. 14 summarizes the sensing response of the testedgases at PEL level. The height of the each bar indicates the sensorresponse in percentage for different analytes. In accordance with anexemplary embodiment, the catalyst embedded SnO₂-SWNT ternarynanostructures shows enhanced sensing performance towards NH₃ and H₂S,with a clear selectivity observed for Pd/SnO₂/SWNT for H₂S gas sensing.

Conclusions:

In accordance with an exemplary embodiment, a simple and facile yetpowerful method for fabrication of hetero-nanostructures and itsapplication towards gas sensing is disclosed herein. A sequentialelectrochemical templating approach was used to functionalize SWNTs withtin oxide nanocrystallites, followed by electrodeposition of metalnanoparticles. Electrical and sensing characterization was used forunderstanding the effect of palladium surface functionalization onSnO₂/SWNT hybrid nanostructures. The palladium decorated SWNT deviceshowed high selectivity towards H₂S gas with a sensor performance of600% observed for 5 ppm concentration. The dramatic enhancement insensing performance observed for Pd decorated SnO₂/SWNT samples wasattributed to the dual role of Pd metal, which served as both catalyticactivator and as Schottky barrier modulator. Additionally by furthercontrolling the amount of loading and the size and growth ofnanoparticles, one can envision the use of such compoundnanoarchitectures for tailored gas sensing applications.

Finally, combined with the already demonstrated ability to make metaland metal oxide functionalized SWNTs, the fabrication of a high-densitygas sensor nanoarrays can be facilitated. The sensor responses obtainedfor different analytes using different sensor recognition elements canpermit subsequent pattern recognition and multi-component analysis innear future.

It will be understood that the foregoing description is of the preferredembodiments, and is, therefore, merely representative of the article andmethods of manufacturing the same. It can be appreciated that manyvariations and modifications of the different embodiments in light ofthe above teachings will be readily apparent to those skilled in theart. Accordingly, the exemplary embodiments, as well as alternativeembodiments, may be made without departing from the spirit and scope ofthe articles and methods as set forth in the attached claims.

What is claimed is:
 1. A sensor having co-functionalized single-walledcarbon nanotube interconnects, the sensor comprising: single-walledcarbon nanotube interconnects; discrete tin oxide nanocrystallitessynthesized onto the single-walled carbon nanotube interconnects; andmetal nanoparticles synthesized onto the discrete tin oxidenanocrystallites and the surface of the single-walled carbon nanotubeinterconnects.
 2. The sensor of claim 1, wherein the sensor comprises aplurality of sensors, which are wire-bonded and each sensor is connectedin series with a load resistor.
 3. The sensor of claim 2, wherein thesensor is a gas sensor.
 4. The sensor of claim 1, wherein the sensor iscomprised of an array of sensors.
 5. The sensor of claim 1, furthercomprising exposing the sensor to an analyte, and wherein the analyte isNH₃, NO₂, H₂S and/or water vapor.
 6. The sensor of claim 1, wherein themetal nanoparticles are palladium (Pd).
 7. The sensor of claim 1,wherein the metal nanoparticles are platinum (Pt).
 8. The sensor ofclaim 1, wherein the metal nanoparticles are gold (Au).
 9. The sensor ofclaim 1, wherein the sensor is co-functionalized for sensing at roomtemperature.
 10. The sensor of claim 1, wherein the sensor is a gassensor.
 11. A sensor comprising: a pair of microelectrodes; and a tinoxide coated single-walled carbon nanotube interconnects, which extendsacross a gap between the pair of microelectrodes, and wherein the tinoxide coated single-walled carbon nanotube interconnects include metalnanoparticles deposited onto discrete tin oxide nanocrystallites and asurface of single-walled carbon nanotube interconnects.
 12. The sensorof claim 11, wherein the discrete tin oxide nanocrystallites issynthesized onto the single-walled carbon nanotube interconnects bypreparing an electrolyte solution comprised of NaNO₃, HNO₃, SnCl₂.5H₂Oand/or other tin ion precursors.
 13. The sensor of claim 12, wherein thediscrete tin oxide nanocrystallites is electrochemical synthesized ontothe surface of the single-walled carbon nanotube interconnects.
 14. Thesensor of claim 11, wherein the metal nanoparticles are palladium (Pd).15. The sensor of claim 11, wherein the metal nanoparticles are platinum(Pt).
 16. The sensor of claim 11, wherein the metal nanoparticles aregold (Au).
 17. The sensor of claim 11, further comprising: a pluralityof sensors, and wherein the plurality of sensors are wire-bonded andeach sensor is connected in series with a load resistor.
 18. The sensorof claim 11, wherein the sensor is co-functionalized for sensing at roomtemperature.
 19. The sensor of claim 11, wherein the sensor is a gassensor.
 20. The sensor of claim 11, wherein the sensor is comprised ofan array of sensors.